Usually, solid oxide fuel cells (SOFCs) employ an electrolyte of ion conductive solid oxide, e.g., stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
Fuel cells of the type described above, solar cells, or the like are used as a direct current power supply for generating direct current power, in an alternating current power supply device for supplying alternating current power to a load, for example. The direct current power generated by the direct current power supply is converted into alternating current power by a converter (direct current to alternating current converter) such as a DC/AC inverter or the like.
Generally, the power conversion efficiency of a DC/AC inverter is maximized when it operates under a nominal load. When the DC/AC inverter operates under a partial load, the power conversion efficiency thereof drops. The drop in the power conversion efficiency tends to adversely affect the efficiency of the overall alternating current power supply device. Structures incorporating a plurality of DC/AC inverters have been adopted in order to improve the adversely affected efficiency.
For example, Japanese Laid-Open Patent Publication No. 61-135364 discloses a low-loss power supply device including, as shown in FIG. 10 of the accompanying drawings, two parallel inverters 3a-1, 3a-2 connected between a direct current power supply 1a and a filter 2a. The inverter 3a-1 has a capacity corresponding to the added capacities of loads such as refrigerators or the like which are operating at all times, and operates at all times. On the other hand, the inverter 3a-2 is controlled by a controller 4a to be operated or shut down depending the load state.
Japanese Laid-Open Patent Publication No. 61-135365 discloses a low-loss power converting device including, as shown in FIG. 11 of the accompanying drawings, a direct current power supply 1b which supplies a direct current via switches 2b-1, 2b-2, 2b-3 to inverters 3b-1, 3b-2, 3b-3. The inverters 3b-1, 3b-2, 3b-3 convert the supplied direct current into an alternating current, which is supplied to loads 4b-1, 4b-2, . . . 4b-n. 
A load detecting circuit 5b detects the magnitudes of loads based on a load current from a current transformer 6b, and supplies the detected magnitudes of loads to a control circuit 7b. The control circuit 7b determines one of the inverters to be operated based on the magnitudes of loads.
Japanese Laid-Open Patent Publication No. 61-135366 discloses a method of controlling a power converting device. According to the disclosed method, as shown in FIG. 12 of the accompanying drawings, a direct current from a direct current power supply 1c is supplies via switches 2c-1 through 2c-3 to inverters 3c-1 through 3c-3, which convert the supplied direct current into an alternating current to be supplied to loads 4c-1 through 4c-n. A load current detector 5c detects a load current supplied to the loads 4c-1 through 4c-n, and inputs the detected load current to a control circuit 6c. 
The control circuit 6c determines the number of inverters to be operated based on the detected load current. The control circuit 6c also refers to operating states of the inverters 3c-1 through 3c-3 which are stored in a storage device 7c, and determines one of the inverters to be operated or shut down next.
Japanese Laid-Open Patent Publication No. 61-273178 discloses a low-loss power converting device including a plurality of parallel inverters, a load detecting means, for detecting whether the load has been activated or not, a load capacity input means, associated with each load, for inputting the capacity of the load each time the load detecting means detects the activation of the load, an adding means for adding output signals from the load capacity input means, and an inverter activating means for determining one of the inverters to be operated based on an output signal from the adding means and activating the inverter.